Fluid flow control or throttle valves are well-known in the art as a means for regulating rates of fluid flow. One type of fluid flow control device is a so-called flapper valve in which a thin "flapper" or plate-like member is disposed inside a fluid passageway and centrally mounted on a rotatable shaft passing laterally through the interior of the passageway. The plane of the flapper can thus be oriented by rotating the shaft in a clockwise or counterclockwise direction. The flapper is precisely dimensioned so as to close and more or less seal the passageway to stop or at least substantially reduce fluid flow when the plane of the flapper is oriented substantially perpendicular to the longitudinal axis of the passageway. Alternatively, rotating the shaft and the flapper 90.degree. or so such that the plane of the flapper is substantially parallel to the longitudinal axis of the passageway results in opening the passageway so as to permit fluid flow. The simplicity and ease of operation of such flapper valves makes them particularly well suited to regulating fluid flow.
For many fluid control applications, such flapper valves can be fabricated using components made of conventional metallic materials such as steel, stainless steel, and so forth. Metallic construction results in valves having a high degree of durability, mechanical strength and good resilience. Where the fluid being regulated, however, is highly corrosive and chemically reactive with conventional metals, for example chlorine and hydrogen fluoride, alternative construction materials must be utilized for control valve fabrication to prevent deterioration of the valve and contamination of the fluid. But, various known corrosion-proof, high-strength alloys and composites are typically extremely expensive, difficult to fabricate, or both. Silicon carbide, for example, is both cost-prohibitive and almost impossible to machine because it is so brittle.
Various relatively inexpensive plastic materials, such as Teflon and related compounds, are known to be highly impervious to corrosive substances such as chlorine and hydrogen fluoride. One solution to this problem would thus be to construct flapper valves in which all components exposed to the regulated corrosive fluid were formed of Teflon or similar corrosion-resistant plastic material. For example, an all-Teflon flapper element could be mounted on an all-Teflon shaft using all-Teflon screws that pass through apertures spaced along the shaft and are received into threaded openings at corresponding locations along one face of the flapper element.
While such a construction would have the desired property of being substantially impervious to corrosive fluids, such a valve would not have a high degree of mechanical strength or a long service life. Because Teflon and similar plastic materials generally have low resilience and relatively low mechanical strength, repeated opening and closing of such an all-Teflon valve would likely lead to early breakage and valve failure. For example, the relatively thin all-Teflon shaft, weakened by the presence of screw openings along its length, would be readily subject to fracture or distortion. Also, the screw elements would have a tendency to work loose from the flapper thereby preventing precision valve operation. An alternative to using screws to secure the flapper to the shaft would be to permanently bond these two elements, for example using an adhesive or ultrasonic welding. But such an alternative construction would make it impossible to disassemble the valve for periodic service and cleaning.
These and other problems and limitations are overcome with the chemically-resistant fluid control valves of this invention.